Microplastics from tyres, highest concentration in urban areas

It is not just smog from exhaust fumes, microplastics are also making themselves felt. Those from tyres, which in urban areas with heavy traffic and where vehicles brake and restart very frequently, can be up to five times higher than in other areas. These are the findings of a study conducted as part of the European Polirisk project with the collaboration of experts from ENEA.

"In the city, one of the main sources of microplastic pollution is the tiny particles generated by the friction of tyres on asphalt during normal vehicle traffic," emphasises Maria Rita Montereali, researcher at ENEA's Territorial Impacts and Developing Countries Laboratory and co-author of the study with her colleagues Laura Caiazzo and Sonia Manzo. "So far, only a few international studies have quantified the atmospheric concentrations of these particles. This scenario is represented by the new study, which, as she points out, measures the presence of microplastics and evaluates the "variations in relation to other primary traffic pollutants, analysing areas with different vehicular traffic conditions". In the future, the data collected can be used to verify possible associations with health effects.

The study, focusing on the period 2022-2023, monitored three sites with different traffic flows and car speeds: an urban road with the 'stop-and-go' type of traffic, a motorway section with high but flowing traffic, and a city park about fifty metres away from the nearest secondary road.

For the air quality measurements, the researchers used markers for synthetic and natural rubber, the main components of tyres, and a marker for benzothiazole, an additive used to harden rubber. "In parallel," emphasise the ENEA, "other traffic-generated pollutants from brake wear, iron, copper, chromium and manganese, and engine emissions were also monitored to check their correlation with the microplastics under study. The lowest concentrations of microplastics from tyres, the highest concentrations were found near the motorway (7.8-18.1 ng/m³) and to a greater extent in the 'stop-and-go' traffic zone where vehicles stopped and restarted frequently. "Thus, compared to the park, the levels of synthetic and natural rubber in the air were on average up to 3 times higher on the motorway and almost 5 times higher in the 'stop-and-go' traffic zone.

Concentrations of benzothiazole (a substance used in the vulcanisation process of rubber for tyre production) were also higher near the motorway (2.4 times higher) and in the 'stop-and-go' zone (4.6 times higher) than in the park. "Although it is also present in other products, such as antifreeze, corrosion inhibitors for the paper industry or intermediates in the synthesis of dyes," continues researcher Laura Caiazzo, "in our study it was considered a marker of particular interest, because it is used in in vitro and in vivo toxicity tests and in our samples it showed a strong correlation with other indicators of the presence of rubber particles in the air. Overall, the study concludes, 'concentrations of metals from brake wear were three to eight times higher in busy areas than in the park. Furthermore, compared to the motorway, the area with intermittent 'stop-and-go' traffic had the highest amounts of metallic elements in the air (2 to 4.8 times higher)'.